Java JIT loop unrolling policy?

Question

What is the loop unrolling policy for JIT? Or if there is no simple answer to that, then is there some way i can check where/when loop unrolling is being performed in a loop?

GNode child = null;
for(int i=0;i<8;i++){
   child = octree.getNeighbor(nn, i, MethodFlag.NONE);
   if(child==null)
      break;
   RecurseForce(leaf, child, dsq, epssq);
}

Basically, i have a piece of code above that has a static number of iterations (eight), and it does bad when i leave the for loop as it is. But when i manually unroll the loop, it does significantly better. I am interested in finding out if the JIT actually does unroll the loop, and if not, then why.

Answer 1

Another way to see if loop unrolling is being performed in the loop is to specify -XX:LoopUnrollLimit=1 as an argument to the JVM when running your code.

If you have an executable jar, then an example of how you can use this is:

java -XX:LoopUnrollLimit=1 -jar your-jar.jar

This flag will

Unroll loop bodies with server compiler intermediate representation node count less than this value

And that'll directly address your question without needing to look at the generated assembly

Answer 2

If the JVM unrolls the loop is probably best answered by actually printing the generated assembly. Note that this requires your code to actually be executed as a hot spot (i.e. the JVM considers it worthy of the expensive optimizations).

Why the JVM decides one way or another is a much harder question and probably requires in-depth analysis of the JIT code.